This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-298278, filed Sep. 29, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an amplifier circuit, particularly to an amplifier circuit including a problem such that a distortion occurs in spread spectrum communication.
2. Description of the Related Art
There will now be described a linear amplifier as one example of a conventional amplifier circuit. The linear amplifier inputs a low level signal, linearly amplifies the signal, and subsequently outputs a desired signal. In general, the linear amplifier comprises a differential amplifier. When the differential amplifier is supplied with a voltage signal and outputs a current signal, a load resistor converts the current signal to a voltage. A current gain xcex94I1 of the differential amplifier in voltage-to-current conversion is represented by the following equation using Vin as an input signal amplitude.                               Δ          ⁢                      xe2x80x83                    ⁢                      I            1                          =                              A            ·            tan                    ⁢                      xe2x80x83                    ⁢                      h            ⁡                          (                                                V                  in                                                  2                  ⁢                                      V                    T                                                              )                                                          (        1        )            
Here A denotes a current value of a constant current source of a differential pair of transistors, and VT denotes a thermal voltage. When tanh␣x is approximated, tanh␣x≈xxe2x88x92x3/3 can be represented. Therefore, the equation (1) can be represented as follows.                               Δ          ⁢                      xe2x80x83                    ⁢                      I            1                          =                  A          ⁡                      (                                                            V                  in                                                  2                  ⁢                                      V                    T                                                              -                                                1                  3                                ⁢                                                      (                                                                  V                        in                                                                    2                        ⁢                                                  V                          T                                                                                      )                                    3                                                      )                                              (        2        )            
Here a second term indicates a distortion component. Particularly a third-order intermodulation distortion (IM3) poses a largest problem as an adjacent channel leakage power of the signal in a spread spectrum radio system. The distortion is generated by presence of the term. Since VT is 26 mV at room temperature, and when IM3 is lowered to xe2x88x9260 dBc or less as a condition for distortion reduction, Vin needs to be 2.8 mV or less. Therefore, the aforementioned linear amplifier has a reduced distortion only when the input signal amplitude is very small. The linear amplifier cannot be utilized in an amplifier in which a signal to be handled is large in a range of 10 mVpp to 1 Vpp, particularly in a power amplifier.
The third-order intermodulation distortion as a main factor of distortion of the amplifier increases when an output power is increased. Therefore, in order to amplify the signal at the reduced distortion, a method of reducing the output power per one stage of the amplifier and obtaining a gain by a multi-stage structure of an amplifier is used. However, this poses problems such as an increase of power consumption, increase of a mounting area by an increase of the number of chips, and cost increase.
It is an object of the present invention to provide an amplifier circuit in which the third-order intermodulation distortion is inhibited without suppressing the output power.
According to a first aspect of the invention, there is provided an amplifier circuit comprising: a differential amplifier configured by a differential pair of transistors; a common emitter amplifier connected in parallel to the differential amplifier and configured by a pair of common-emitter configuration transistors; input and output terminals which are common to the differential amplifier and the common emitter amplifier, an input signal being input to the input terminals and an output signal output from the output terminal; and a bias controller configured to control a bias of at least one of the differential amplifier and the common emitter amplifier.
According to a second aspect of the invention, there is provided an amplifier circuit comprising: a first amplifier whose input-to-output characteristic indicates a hyperbolic tangent function characteristic; a second amplifier whose input-to-output characteristic indicates an exponential characteristic, the second amplifier being connected in parallel to the first amplifier; input and output terminals which are common to the differential amplifier and the common emitter amplifier; and a bias controller configured to control a bias of at least one of the first and second amplifiers.